Multi-shelled Cr2O3 hollow microspheres for high-performance lithium-ion battery anode materials

To date, the problems of energy depletion and environmental pollution have become more and more rigorous. One effective method to address these problems is developing novel, pollution-free energy storage and conversion systems. Wherein, rechargeable lithium-ion batteries (LIBs) due to their obvious advantages such as light-weight, memory-effect-free, and environmental-benignity, have become the dominant power sources for portable electronic devices and even taken a place in pure electrical vehicles. However, the further development of next-generation LIBs to better satisfy the requirements of the economics and the environment is largely hindered by the low accessible energy density. Transition metal oxides have been found to exhibit a high theoretical capacity. However, the plateau is also very high (1.1, 1.0 and 1.2 V for Co3O4, Fe2O3 and NiO, respectively), inducing a narrow potential window, thus the energy density is still low. Conversely, Cr2O3 possessing not only a high theoretical capacity (1058 mAh/g) but also a relatively low lithium-insertion voltage (about 0.1-0.4 V vs. Li+/Li), has been considered as a promising anode material of LIBs for a long time. However, the actual application of Cr2O3 still suffers from poor cycling performance caused by the huge volume variation, structural pulverization and electrical contact loss during the lithium insertion/extraction processes. As previously reported, replacing the bulk structures with hollow and/or porous micro-/nanostructures as the electrode materials can settle down the above issues. That is because: (1) Shorter electron and ion transition path, faster lithium insertion/extraction, thus higher rate capability; (2) the interior hollow space can buffer the volume change, thus guaranteeing stable structure, indicating a better cycling performance; (3) hollow micro-/nanostructure usually possesses a higher specific surface area, indicating more surface lithium-storage sites and resulting in a higher capacity. Moreover, well designed three-dimensional multi-shelled hollow microspheres composed of 0D nanoparticles with the shells supporting for each other, can better alleviate the stress and strain, leading to further improved structural stability and insuring good electrical contact. Given the above considerations, many efforts have been focused on the synthesis of Cr2O3 hollow or porous nanostructures. However, the synthesis of multi-shelled Cr2O3 hollow microspheres seems difficult, which is due to that Cr precursors (that's Cr (III) and Cr (VI) ions) mainly exist as anions. Cr (VI) ions almost always exist as anions in aqueous solutions, while Cr (III) ions will precipitate out forming Cr(OH)(3) at near neutral to alkaline solution. As a result, it seems quite challenging to synthesize multi-shelled Cr2O3 hollow microspheres. In this communication, we reported the synthesis of multi-shelled Cr2O3 hollow microspheres by adsorbing CrO42- anions into negatively-charged carbon microsphere (CMS) templates, followed with a catalytic combustion process. Multishelled Cr2O3 hollow microspheres with a shell number as many as five were successfully synthesized through a general hard-template method. The possibility of adsorbing anions through negatively-charged CMS templates was verified clearly. Besides, the shell number as well as porosity of the hollow microspheres was well controlled. These structural parameters are found to have a significant impact on the lithium-storage properties. When tested as the anode of LIBs, theses multi-shelled Cr2O3 hollow microspheres exhibited a much better LIB performance than Cr2O3 nanoparticles, showing a super-high specific capacity, excellent cycling stability, and notable rate capability. Wherein, the quadrupleshelled showed the best performance, achieving a new record high capacity of 1031.2 mAh/g after 100 cycles. The impressive LIB performance was benefited from the multi-shelled hollow microstructures, which guarantees enhanced reactivity, more lithium-storage sites, shorter Li ions and electron diffusion length as well as additional void space to butter the volume expansion. Given the highly-controllable synthesis method and significantly improved performance, one can expect that these Cr2O3 hollow microspheres can be promising candidate anode materials for high-performance LIBs.